Abstract
Endogenous and environmental variables are fundamental in explaining variations in fish condition. Based on more than 20 yr of fish weight and length data, relative condition indices were computed for anchovy and sardine caught in the Gulf of Lions. Classification and regression trees (CART) were used to identify endogenous factors affecting fish condition, and to group years of similar condition. Both species showed a similar annual cycle with condition being minimal in February and maximal in July. CART identified 3 groups of years where the fish populations generally showed poor, average and good condition and within which condition differed between age classes but not according to sex. In particular, during the period of poor condition (mostly recent years), sardines older than 1 yr appeared to be more strongly affected than younger individuals. Time-series were analyzed using generalized linear models (GLMs) to examine the effects of oceanographic abiotic (temperature, Western Mediterranean Oscillation [WeMO] and Rhone outflow) and biotic (chlorophyll a and 6 plankton classes) factors on fish condition. The selected models explained 48 and 35% of the variance of anchovy and sardine condition, respectively. Sardine condition was negatively related to temperature but positively related to the WeMO and mesozooplankton and diatom concentrations. A positive effect of mesozooplankton and Rhone runoff on anchovy condition was detected. The importance of increasing temperatures and reduced water mixing in the NW Mediterranean Sea, affecting planktonic productivity and thus fish condition by bottom-up control processes, was highlighted by these results. Changes in plankton quality, quantity and phenology could lead to insufficient or inadequate food supply for both species.
Highlights
Body condition is widely used in ecological studies to determine the nutritional or physiological status of an individual (Bolger & Connolly 1989, Stevenson & Woods 2006) and is defined as the quantity of nutrient reserves, i.e. the quantity of accumulated energy reserves exceeding that required for daily nutritional demands (Schulte-Hostedde et al 2001, Schamber et al 2009)
The period and the interaction between period and month were significant for sardine (GLMs; df = 1, p < 0.001 and df = 11, p < 0.001, respectively), indicating changes in the phenology of the condition and the need to run separate analyses for each period
Anchovy showed a minimal Kn in winter during January (0.89 ± 0.002) and February (0.89 ± 0.003), and a peak at the beginning of summer in June (1.05 ± 0.002) and July (1.05 ± 0.001)
Summary
Body condition is widely used in ecological studies to determine the nutritional or physiological status of an individual (Bolger & Connolly 1989, Stevenson & Woods 2006) and is defined as the quantity of nutrient reserves, i.e. the quantity of accumulated energy reserves exceeding that required for daily nutritional demands (Schulte-Hostedde et al 2001, Schamber et al 2009). Individuals with larger nutritional reserves may have a higher probability of survival and reproductive success or growth (Millar & Hickling 1990), which may in turn affect the dynamics of the whole population (Jakob et al 1996, Adams 1999). Anomalous conditions have been shown to strongly affect spawning and egg quality and quantity in many fish species (Adams 1999, Óskarsson et al 2002). Earlier sexual maturation, associated with smaller size as well as higher natural mortality, has been shown to occur in Atlantic herring Clupea harengus in poor condition (Winters & Wheeler 1994, Heino & Godø 2002, Malzahn et al 2007), while the chances of becoming mature increase with good condition in American plaice Hippoglossoides platessoides (Morgan 2004)
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